This paper presents a system-level codesign methodology of hardware/software for HDTV source decoder. We give out the hardware/software codesign method, which can meet the requirements of HDTV source decoding application. According to the algorithm development and analysis, the MPEG-2 MPHL decoder process is partitioned into software and hardware at the system level. We construct the system model that can represent the system architecture and application program of interest. A platform-based design environment is developed that can provide modularity, scalability, and flexibility in cosimulation and development for system on chip. Our main contribution is to introduce platform-based codesign tools where the software and hardware are validated and developed concurrently at the cycle accurate level.